1. Field of the Invention
This invention relates to an image recording apparatus and method using a laser beam as a light source, especially to an image recording apparatus and method for improving a density uniformity of a recorded image in the main scanning direction.
2. Description of the Prior Art
In an image recording apparatus using a laser beam as a light source, a photosensitive material such as a film is scanned in the main scanning direction by use of a laser beam which is deflected in a one-dimensional direction by a polygon mirror or the like, while the photosensitive material is relatively moved in a sub scanning direction. In this way, a two dimensional scanning is done and an image data is recorded. The sub scanning direction is perpendicular to the main scanning direction.
Such an apparatus generally employs an f.multidot..theta. lens for image focusing. A beam spot is formed on the photosensitive material by the f.multidot..theta. lens, and then an image is recorded on the photosensitive material by a modulated laser beam. The modulation of the laser beam is done directly if the laser beam is emitted from a laser diode, or done by an AOM (acoustic-optical modulator) or the like if a gas laser beam is used.
The laser beam is modulated by an image signal in synchronization with a reference clock which is generated at a certain frequency each time the laser beam is detected by a sensor for detecting a scanning starting in the main scanning direction.
If an f.multidot..theta. lens is used for image focusing and a deflector whose reflective surface is rotated at a fixed angular velocity such as a polygon mirror is used for scanning in the main scanning direction, the scanning speed at the beam spot is fixed owing to the f.multidot..theta. characteristic of the lens. In other words, if an image is recorded on the photosensitive material in synchronization with a reference clock having a fixed frequency when the photosensitive material is on the image focusing surface, a uniform pixel density is obtained at any position in the main scanning direction. Accordingly, the obtained image has substantially no pixel density unevenness in the main scanning direction.
The above theory will be explained with reference to equations.
An incident angle 8 of the laser beam on an f.multidot..theta. lens has the following relationship with a height.times.of the image. EQU x=f.multidot..theta. (1)
The above equation (1) is time-differentiated into: EQU dx/dt=f.multidot.d.theta./dt (2)
The left side of the equation (2) corresponds to a scanning speed of the laser beam on the image focusing surface, and d.theta./dt of the right side thereof corresponds to an angular velocity of the polygon mirror (namely, the change of the above incident angle .theta. by the passage of time).
A fixed angular velocity of the polygon mirror means a fixed value of d.theta./dt in an area where an image is recorded validly in the main scanning direction. (Such an area will be referred to simply as the valid area.) Accordingly, the scanning speed dx/dt is fixed, resulting in an image having a uniform pixel density in the main scanning direction.
Today, a new f.multidot..theta. lens is demanded which reduces a diameter of the beam spot and also increases a scanning angle. The reduced diameter of the beam spot raises a recording density, and the increased scanning angle enlarges the valid area.
An f.multidot..theta. lens, which is produced especially to meet the above demands, is somewhat inferior in the above-mentioned advantageous f.multidot..theta. characteristic. In other words, since the relationship expressed in the equation (1) is not fully obtained, the scanning speed at the beam spot cannot be fixed even if the angular velocity of the polygon mirror is fixed. The result is a recording density unevenness of the obtained image.